1. Field of the Invention
The present invention relates to an injection molding method for manufacturing a molded product by injecting resin from a plurality of injection units.
2. Description of the Related Art
Conventionally, a molded product obtained from injection molding is manufactured by press-fitting resin in molten state into a cavity and then cooling and solidifying it, but if the inner part of the molded product is placed in the state of negative pressure due to the volumetric reduction of the resin at the time of solidification, sink marks, which are dimples formed because the skin part is drawn into the inner part, are produced. Therefore, in order to prevent this problem, various injection molding methods have been practiced from the past for preventing the inner part of the molded product from being placed under a negative pressure at the time of solidification.
In particular, following methods have been practiced: (1) a method wherein a predetermined amount of resin is injected from an injection unit into a cavity and then the resin is additionally injected again from the injection unit, whereby the volumetrically reduced quantity at the time of solidification may be compensated, (2) a method wherein resin is injected from the injection unit into the cavity and then gas such as air, nitrogen, or the like is introduced into the portions at which sink marks may be easily formed, and (3) a method wherein foaming agent is added to resin, whereby the volumetrically reduced quantity at the time of solidification is compensated with the expansion of the foaming agent.
However, with the first method in which the resin is added as in the above prior art, the added amount is apt to be surplus because a small amount of resin is added using an injection unit that is set to inject a large amount of resin into a cavity. A problem is also caused in that if the added amount is surplus, the weight of molded product is increased more than is necessary and the cost of material is increased because the amount of resin to be used is increased. Furthermore, another problem will be caused in that because the pressure of resin is increased as a whole, the metal mold may be opened whereby badness of molding or burr may be produced or the metal mold becomes unable to be used within a short time.
Also, with the second method in which gas is introduced into the predetermined portions in the cavity, there is a problem in that it is required to provide a gas introduction path for leading the gas into the predetermined portions, whereby the cost of the metal mold arrangement can be increased or sometimes it is impossible to provide such a gas introduction path in the metal mold arrangement in accordance with the shape of molded products. Furthermore, with this method, there are problems in that the strength of molded products is apt to be deteriorated because hollows are produced within the molded products due to the gas, and in that shapes of the molded products and usable resins can be limited because the gas can be introduced into even an unnecessary portion in the case of a soft resin such as a polypropylene resin or the like.
Still yet, with the third method in which a foaming agent is added into the resin, there are problems in that the cost of material is increased due to the foaming agent while the strength of molded products is decreased due to hollows formed within the molded products by the foaming agent, and in that gas traces (small mark) are produced on the surfaces of the molded products.
Therefore, it is an object of the present invention to provide an injection molding method and an injection molding apparatus, which can precisely perform compensation of resin for the volumetrically reduced quantity without supplying gas or adding foaming agent, which causes various problems as, explained in the above.
In order to achieve the above object, the present invention provides an injection molding method for obtaining a molded product consisting of a skin layer and a core layer by injecting a skin layer resin and a core layer resin from a skin injection unit and a core injection unit, respectively, into a metal mold, comprising the steps of:
injecting said skin layer resin from said skin injection unit into said metal mold, wherein the value of injection pressure is a predetermined value of the initial skin layer injection pressure;
injecting said core layer resin from said core injection unit into said metal mold after a predetermined length of time is passed from the start of injection of said skin layer resin, wherein the value of injection pressure is a predetermined value of the initial core layer injection pressure and said initial core layer injection pressure value is greater than the value of said initial skin layer injection pressure;
changing said skin layer resin injection pressure to a predetermined value of the skin layer dwelling pressure after a predetermined length of time is passed from the start of injection of said core layer resin, wherein the value of said skin layer dwelling pressure is smaller than the value of said initial skin layer injection pressure;
changing said core layer resin injection pressure to a predetermined value of the core layer resin dwelling pressure after a predetermined length of time is passed from the time that the said skin layer resin injection pressure is changed to the value of said skin layer dwelling pressure, wherein the said core layer dwelling pressure is smaller than the value of said initial core layer injection pressure and greater than the value of said skin layer dwelling pressure; and
maintaining the state that the value of said core layer dwelling pressure is greater than the value of said skin layer dwelling pressure over a predetermined length of time.
According to the above constitution, when the skin layer resin for forming the skin layer is cooled and solidified from the surface side to the inner side and thus a volumetric reduction is induced, the core layer resin injected to the core layer disposed inside the skin layer is existent in a pressure higher than the pressure of the skin layer resin so as to compensate the volumetrically reduced quantity, whereby the drop of internal pressure induced by the volumetric reduction of skin layer resin will be prevented. Due to this, when the skin layer resin and the core layer resin are cooled and solidified, and become a molded product, neither sink marks are produced on the surface of the molded product nor hollows are formed in the molded product.
In the above injection molding method, the value of said initial core layer injection pressure is set to be larger than the value of said initial skin layer injection pressure so as to be sufficient to compensate the volumetrically reduced quantity of said skin layer resin at the time of solidification of said skin layer resin. Alternatively, the value of said core layer dwelling pressure may be set to be larger than the value of said skin layer dwelling pressure so as to be sufficient to compensate the volumetrically reduced quantity at the time of cooling and solidification of said skin layer resin.
Moreover, in the above injection molding method, the value of said initial core layer injection pressure and the value of said core layer dwelling pressure are set to be larger than the value of said initial skin layer injection pressure value and the value of said skin layer dwelling pressure, respectively, so that the volumetrically reduced quantity at the time of cooling and solidification of said skin layer resin can be sufficiently compensated. According to this, it is possible to more certainly prevent sink marks and hollows of the molded product.
Here, it is very suitable that said core layer dwelling pressure is developed by the injection pressure of said core injection unit. According to this, it is not required to perform a complex treatment such as handling the metal mold or the like.
Furthermore, in the above injection molding method, it is very suitable that the injection capacity of said core injection unit is equal to or less than the injection capacity of said skin injection unit.
According to this, it is possible to finely adjust the amount of the core layer resin to be injected, whereby the core layer resin can be precisely injected in a required minimum amount which will not generate sink marks nor hollows of molded product. Therefore, it is possible to suppress the increase of excessive weight of molded product and the increase of cost caused thereby.
In addition, in the above injection molding method, it is very suitable that the amount of said core layer resin to be injected is smaller than the amount of said skin layer resin to be injected.
According to this, the excessive use of the core layer resin which leads to the increase of weight and cost can be sufficiently suppressed.
In addition, in the above injection molding method, it is possible that the value of said skin resin injection pressure by said skin injection unit may be reduced step by step after the start of injection of said core layer resin from said core injection unit into said metal mold.
Also, in the above injection molding method, it is possible that the injection pressure value of skin resin by said skin injection unit may be set to a pressure value smaller than the value of said skin layer dwelling pressure before changed to the value of said skin layer dwelling pressure.
Further, in the above injection molding method, it is possible that at least one of the value of said core layer resin injection pressure and the value of said skin layer resin injection pressure may be reduced step by step after the state that the value of said core layer dwelling pressure is larger than the value of said skin layer dwelling pressure is maintained over a predetermined length of time.
Still, in the above injection molding method, it is very suitable that said core layer resin and said skin layer resin are homogeneous or have compatibility while being heterogeneous. According to this, the core layer and the skin layer can be sufficiently adhered, thereby increasing the strength of molded product.
Still yet, the present invention provides an injection molding apparatus for obtaining a molded product consisting of a skin layer and a core layer comprising:
a metal mold;
a skin injection unit for injecting a skin layer resin into said metal mold;
a core injection unit for injecting a core layer resin into said metal mold, wherein the injection capacity of said core injection unit is equal to or less than the injection capacity of said skin injection unit;
a skin layer resin injection pressure control means for controlling the magnitude of the injection pressure of said skin layer resin;
a core layer resin injection pressure control means for controlling the magnitude of the injection pressure of said core layer resin; and
an injection control device for controlling said skin injection unit, said core injection unit, said core layer resin injection pressure control means, and said skin layer resin injection pressure control means to perform a injection molding method as defined above.
By using the apparatus having the above constitution, it is possible to cheaply produce a molded product without generating sink marks or hollows.
Furthermore, the present invention provides a molded product formed by the above injection molding method. Such a molded product is suitable for transparent components of polycarbonate, acryl resin and the like, or components that require high flatness such as compact disc (CD), in addition to being applicable to components for automobile.